1. Field of the Invention
The invention relates to an apparatus for insulating and sealing electrode holders in a reactor for depositing polycrystalline silicon, and to a process for producing polycrystalline silicon using such an apparatus.
2. Description of the Related Art
High-purity silicon is generally produced by the Siemens process. This comprises introducing a reaction gas comprising hydrogen and one or more silicon-containing components into a reactor fitted with support bodies heated by direct passage of current, upon which solid silicon is deposited. Preferably employed silicon-containing compounds are silane (SiH4), monochlorosilane (SiH3Cl), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), tetrachlorosilane (SiCl4) and mixtures thereof.
Each support body is generally composed of two thin filament rods and one bridge which generally connects adjacent rods at their free ends. The filament rods are most commonly fabricated from mono- or polycrystalline silicon, metals/alloys or carbon being employed more rarely. The filament rods are slotted vertically into electrodes located on the reactor floor which provide the connection to the electrode holder and current supply. High-purity polysilicon is deposited on the heated filament rods and the horizontal bridge to increase the diameter thereof over time. The process is terminated once the desired diameter has been achieved.
The silicon rods are held in the CVD reactor by special electrodes generally made of graphite. In each case two filament rods having different voltage polarities at the electrode holders are connected by a bridge at the other slim rod end to form a closed electrical circuit. Electrical energy for heating the slim rods is supplied via the electrodes and their electrode holders. This causes the diameter of the slim rods to increase. The electrode simultaneously grows into the rod base of the silicon rods, starting at its tip. Once a desired target diameter for the silicon rods has been achieved the deposition process is terminated and the silicon rods are cooled and removed.
The sealing of the electrode holder which passes through the floor plate is of particular importance.
Escaping chlorosilane reacts with the oxygen and the moisture in the surrounding air to form silica and HCl which, with further moisture, condenses in the form of aqueous HCl. The reaction products silica and aqueous HCl and also corrosion products generated by HCl are deposited at the end of the feedthrough of the electrode holder through the floor plate and bridge the insulation of the electrode holder to the floor plate, thus resulting in a ground fault and outage of the deposition reactor.
Since the reaction products accumulate at the location of formation at a desired operating time of the deposition reactor (service life between 2 overhauls) spanning several months even small leaks and leakage streams can result in premature failure through ground faults. When considering leakage streams it is necessary to consider not only leakage at the sealing surfaces but also leakage by diffusion through the sealing material itself.
The use of sealing bodies has been proposed to this end, importance attaching in particular to the arrangement and shape of the sealing bodies and the sealing material employed.
Located between the top of the electrode holder, which protrudes into the deposition equipment, and the floor plate is an annular body. This body typically has two functions: 1) sealing of the electrode holder feedthrough and 2) electrical insulation of the electrode holder from the floor plate.
The high gas-space temperature in the CVD reactor necessitates thermal protection of hydrocarbon-based sealing bodies. Insufficient thermal protection results in premature wear of the sealing bodies due to scorching of the sealing bodies, thermally induced flow of the sealing body, reactor leaks, the distance between electrode holder and floor plate falling below the minimum value, and ground faults at charred sealing bodies. Ground faults or leaks result in outage of the deposition equipment and hence in the deposition process being aborted. This results in reduced output, a lower yield due to material degradation and higher costs.
US 20110305604 A1 discloses shielding the electrode seals from thermal stress using protective rings made of quartz. The reactor floor has a special configuration. The reactor floor comprises a first region and a second region. The first region is formed by a plate facing toward the interior of the reactor and an intermediate plate carrying the nozzles. The second region of the reactor floor is formed by the intermediate plate and a floor plate carrying the supply connections for the filaments. The cooling water is fed into the first region thus formed in order thus to cool the reactor bottom. The filaments themselves are seated in a graphite adapter. This graphite adapter engages with a graphite clamping ring, which itself interacts with the plate via a quartz ring. The cooling water connections for the filaments may be in the form of quick-fit couplings.
WO 2011092276 A1 describes an electrode holder where the sealing element between the electrode holder and the floor plate is protected against the effects of temperature by a circumferential ceramic ring. A plurality of electrodes are secured in a floor of the reactor. These electrodes carry filament rods seated in an electrode body which supplies current to the electrodes/filament rods. The electrode body itself is mechanically prestressed in the direction of the top face of the floor of the reactor by a plurality of resilient elements. A radially circumferential sealing element is inserted between the top face of the floor of the reactor and a ring of the electrode body which is parallel to the top face of the floor. The sealing element itself is shielded by a ceramic ring in the region between the top face of the floor of the reactor and the ring of the electrode body which is parallel thereto.
The sealing element is made of PTFE and assumes both the sealing function and the insulating function. The ceramic ring serves as a heat shield for the sealing ring.
However, subjecting PTFE to thermal stress above 250° C. results in scorching/cracking at the sealing surface and flow of the sealing body. The distance between the top of the electrode holder and the floor plate thus falls below a minimum value leading to electrical arcing/ground faults from the electrode holder to the floor plate. The scorching/cracking also releases carbon compounds which lead to contamination of the silicon rods to be deposited due to incorporation of carbon.
US 20130011581 A1 discloses an apparatus for protecting electrode holders in CVD reactors which comprises an electrode which is suitable for accommodating a filament rod and is disposed atop an electrode holder made of an electrically conductive material and mounted in a recess in a floor plate, wherein an intermediate space between the electrode holder and the floor plate is sealed with a sealing material and the sealing material is protected by a protective body constructed from one or more parts and arranged in a ring shape around the electrodes, wherein the height of the protective body increases at least in sections in the direction of the electrode holder.
The document provides for geometrical bodies arranged concentrically around the electrode holder, their height decreasing with an increasing distance from the electrode holder. The body may also be composed of one part. This provides for thermal protection for the sealing and insulating body of the electrode holder and also for flow modification at the rod base of the deposited polysilicon rods, which has a positive influence on the fallover rate.
The apparatuses according to WO 2011092276 A1 and according to US 20130011581 A1 can suffer from ground faults between the electrode holder and the floor plate due to silicon slivers which, on account of thermal stresses due to the high feed rate, spall off the silicon rods, fall between the electrode holder and the ceramic ring/support body and there produce an electrically conducting connection between the electrode holder and the floor plate. Short circuits entail abrupt process termination due to outage of the current supply for heating the rods. The rods cannot be deposited up to the intended end diameter. Thinner rods lead to lower plant capacity which results in considerable costs.
CN 202193621 U discloses an apparatus providing two ceramic rings between the top of the electrode holder and the floor plate with a graphite gasket located between them.
However, this apparatus provides no sealing function between the ceramic ring and the top of the electrode holder nor between the ceramic ring and the floor plate. The reactor consequently suffers from leaks.
CN 101565184 A discloses an insulating ring made of zirconium oxide ceramics material (ZrO2) between the top of the electrode holder and the floor plate. The insulating ring is recessed in the floor plate. An additional quartz ring is therefore required for insulation between the top of the electrode holder and the floor plate. Sealing is achieved via two graphite gaskets between the top of the electrode holder and the insulating ring and between the floor plate and the insulating ring. An O-ring is employed at the electrode feedthrough below the floor plate as a further seal.
CN 102616783 A discloses an insulating ring made of ceramics material between the top of the electrode holder and the floor plate. Sealing is achieved via two metal framed graphite gaskets above and below the insulating ring toward the top of the electrode holder and toward the floor plate respectively.
The problem with the embodiments described in the latter two documents is that the graphite gasket requires high contact pressures to achieve sealing. Since ceramics material is brittle and has a low flexural strength the sealing surfaces of the floor plate and the top of the electrode holder are subject to strict evenness requirements. The slightest unevenness which is almost unavoidable in practice and high clamping torques result in high point loading and fracturing of the ceramic rings. The reactor consequently suffers leaks.
WO 2014/143910 A1 discloses a sealing ring between the floor plate and the electrode holder comprising a base body made of a ceramic material with an upper and a lower groove, wherein sealing elements are inserted into the respective grooves.
However, two opposite grooves weaken the mechanical properties of the insulating ring. Upon compression, in particular for uneven sealing surfaces, this can lead to fracturing of the insulating ring. The cutting of the grooves in the ceramic material of construction with the accompanying constructional standards (dimensional accuracy, low surface roughness) is very costly. The demands on the mating sealing surface (top of the electrode holder and floor plate) are likewise very high. This causes additional costs.
US 2010058988 A1 provides for securing the electrode holder in the floor plate via a conical PTFE sealing and insulating element. The top face of the conical PTFE sealing element is compressed against the electrode holder via a flange (cross-sectional widening). An O-ring is additionally provided both between the sealing element and the electrode feedthrough through the floor plate and between the sealing element and the shaft of the electrode holder.
The compression of the conical sealing element impedes removal of the electrode holder. Flow of the PTFE sealing body can result in the distance between the electrode holder and the floor plate falling below the minimum value. This results in electrical arcing/ground faults.